This question can be answered through the way in which Don John manages to convince both his brother, Don Pedro, and Claudio of Hero's guilt. Note how even in Act III scene 3, even before Don John has shown his brother and Claudio "proof" of Hero's guilt, Claudio and Don Pedro seem to believe him and Claudio is already planning on how he will use his wedding day to "shame" Hero for her wantonness:

If I see anything tonight why I should not marry her, tomorrow, in the congregation where I should wed, there will I shame her.

In one sense, the audience cannot help but judge Claudio very strongly for this statement and the way he subsequently names and shames Hero in Act IV scene 1, the wedding scene, so cruelly. However, arguably, Shakespeare uses this and Don John to highlight the way in which the character of a woman, once besmirched and tainted with any rumour of sexual impropriety, however fabricated, is lost forever. It is this that the character of Don John is used to explore. All it takes is one word from him and Don John has already managed to make Don Pedro and Claudio doubt the nature of Hero as a woman, even though she has given them no cause whatsoever, unlike Don John, whose past treachery should at least make them doubt what he says. When Don John is able to "show" them Hero being unfaithful, her doom is sealed, and it is notable that Hero is only able to be resurrected as a "cousin" rather than her own person. Don John is a character in this play that Shakespeare uses to explore Renaissance attitudes to women and honour, and how precarious such states actually are.